Recent higher integration and speed of LSI integrated circuits has required more advanced micropatterning in patterning conditions and thus high-resolution resist patterns. In addition to lithography characteristics such as pattern shapes, contrast, mask error enhancement factor (MEEF), and roughness in particular, the improvement in defects (surface defect) of developed resist patterns has been more increasingly urgent than conventionally required. Such defects are known as faults detected with surface defect viewing instrument (“KLA”, Product from KLA-Tencor Corporation) that observes developed resist patterns from the above, such as scum, bubbles, dust, and bridges between resist patterns in developed resist patterns. The resulting defects are partially attributed to low solubility to a cast solvent in a resist composition such as a photo acid generator and residual defects after the use of a developer.
One common means for reducing defects caused by the use of a photo acid generator is to improve the solubility to a developer and a resist solvent by modification from a photo acid generator. The improvement in solubility to an organic solvent may require introduction of a linear alkyl group, a fluorine atom and others, while the alkaline developer solubility can be improved by introducing a polar group such as a hydroxy group. A known effective method for improving the solubility to a solvent is to introduce a hexafluoroalcohol unit, e.g., those having excellent compatibility in the following sulfonium cation illustrated in Patent Documents 1 and 2. As a resist composition, the use of the photo acid generators disclosed in these documents can certainly reduce defects, but it is very hard to obtain satisfactory lithography performance due to swelling and increasing acid diffusion length. Likewise, anion modification is expected to improve the compatibility, while the resulting acid diffusion promotion can degrade lithography performance,
